In pump units, such as distributor injection pumps of fuel injection systems in motor vehicles, for example, it is important to ensure a safe venting of the pump unit. For example, a distributor injection pump is vented when the pump is started. Aside from that, when the fuel tank has run empty, air can also be sucked into the distributor injection pump and must then be allowed to escape from the delivery chambers of the distributor injection pump before any fuel can be made to flow.
In a conventional fuel injection pump of this type, described in, for example, German Patent No. DE-OS 25 22 374, recesses in the form of connecting cross sections have been machined into the cylindrical surface of the pump plungers. These recesses extend from the outlet orifices of the discharge channel, starting from the side of the pump working chamber. The recesses can have a rectangular contour, can have different widths in the peripheral direction of the pump plunger and can also differ from one another in their axial dimension, i.e., their length. The purpose of a configuration of this type is to achieve a cross-sectional profile that bends during the opening stroke of the pump plunger. The intent is, after an initially throttled pressure relief, to enlarge the pressure relief cross section via one of the connecting openings by the addition of the second connecting opening. The purpose of these connection cross sections is, in particular, to eliminate the throttling effect that occurs at different rotational speeds of the fuel injection pump. These cutoff bores are provided in particular to adjust the injected fuel quantity as a function of the rotational speed. In this context, one of the connecting cross sections is generally realized in the form of a throttle slot. One of the requirements for self-igniting internal combustion engines, when they are operating in the low-load range, and particularly at idle, is that the fuel must be injected into the combustion chamber in a precisely timed manner, but with an extended injection period. This method prevents “knocking” of the internal combustion engine, which is particularly noticeable when the engine is operating at idle. The purpose of the extended injection period is to ensure that the quantity of fuel injected during the ignition lag does not become too great, and therefore to ensure that too much fuel is not ignited suddenly, which would lead to a sudden increase in pressure which causes knocking.
German Patent No. DE 36 44 150 describes a fuel injection pump for internal combustion engines. This pump has a pump cylinder which is both reciprocating and rotates, and can therefore be used as a distributor of the fuel delivered to a plurality of pump plungers that supply injection points. The pump plunger delimits a pump working chamber in the pump cylinder. The quantity of fuel delivered by the pump plunger is controlled by varying the opening of an outlet orifice on the pump plunger periphery of a discharge channel that is located in the pump plunger and leads from the pump working chamber to a discharge chamber using an annular slide valve that can be moved axially on the pump plunger by an injected-fuel quantity regulator inside the discharge chamber. The annular slide valve has a control edge and at least two connection cross sections of different shapes situated in the connection between the outlet orifice and the connection to the discharge chamber created during the pump plunger delivery stroke by the control edge on the annular slide valve. One of the connection cross sections has a reduced cross section that has a throttling action and is connected first with the discharge chamber during the pump plunger delivery stroke and before another non-throttling connection cross section which has a larger cross section.
European Patent No. EP 0 323 984 describes a fuel injection system for internal combustion engines. This system includes a high-pressure pump that delivers a specific quantity of fuel per pump cycle from a pump working chamber using a first control valve that is located in a first discharge channel, controls a first return quantity, and determines, in particular, the beginning and end of the delivery of the fuel injection. Also provided are a metering port having a constant cross section, and an electrically controlled second control valve which is connected in series thereto and is located in a second discharge channel for a second return quantity. An electronic control unit is used to process the characteristics of the internal combustion engine and of the fuel injection pump into the control variables that regulate the injection. In the second discharge channel, a differential-pressure gauge is provided to measure the quantity and has an element which is flexibly positioned against a restoring force, and is pressurized against the restoring force on the one hand by the pressure on the working-chamber-side of the pump upstream of the metering port, and, on the other hand, by the discharge-side pressure downstream of the metering port. Its excursion is measured by a travel sensor as a characteristic of the differential-pressure gauge. In the electronic control unit, in addition to the characteristics of the differential-pressure gauge and of the second control valve, the quantity of fuel flowing out via the second discharge channel is determined in the form of a control value, and the control time of the first control valve is modified on the basis of this control value.